feat: detect webxdc by application/webxdc+zip MIME type

This is simpler and can count as hardening
because we don't call async_zip parser anymore
just to check if received file is a webxdc.
This way if ZIP parser is buggy and e.g. panics,
or gets into infinite loop, or is accidentally quadratic,
or allocates a lot of memory, this cannot be triggered
by just sending a file.

test-data/message/webxdc_bad_mime_type.eml

@@ -10,9 +10,8 @@ Content-Type: multipart/mixed; boundary="==BREAK=="
 --==BREAK==
 Content-Type: text/plain; charset=utf-8
 
-webxdc with good extension;
-the mimetype is ignored then,
-content is checked.
+webxdc with bad mime type,
+not detected as a webxdc
 
 --==BREAK==
 Content-Type: text/html